Research On Synthesis,Modification And Electrochemical Performances Of Vanadates As Novel Anode Materials For Lithium-ion Batteries

In this paper,vanadates（Fe,Zn）as anode materials for lithium-ion batteries were studied.The synthesis,crystal structure,morphology and electrochemical performances of the prepared samples were mainly investigated and discussed.The some important conclusions were drawn from our experiments.The main contents were listed as follows:（1）The precursors of FeVO4·xH2O and FeVO4·xH2O/graphene composite were synthesized through a hydrothermal method by using Fe（NO₃）₃·9H₂O,NH₄VO₃ and graphene oxide（GO）as raw materials.XRD,TG,Raman,SEM and TEM techniques were used to characterize the structure,thermostability,composition and morphology.The formation process of the prepared samples was also explained.The results showed that the prepared samples took FeVO4·xH2O as main structure,which owned the morphology of microrods.After being combined with graphene,the crystallinity of the sample got improved markedly and the microrods were evenly anchored in the internet of graphene.Besides,the size of the samples decreased significantly.The electrochemical properties were tested by galvanostatic charge/discharge system and electrochemical workstation.In addition,the electrochemical reaction mechanism was also studied.The discharge specific capacity of FeVO4·xH2O and FeVO₄·xH₂O/graphene composite still maintained at 487.7 mAh g^-1 and 1389.6 mAh g^-1respectively after 100 cycles at the current density of 100 mA g^-1 between 0.01 and 3.0 V versus Li/Li⁺.Via comparison,the FeVO₄·xH₂O/graphene composite behaved the lower charge transfer resistance and the higher Li⁺diffusion coefficient.Therefore,the FeVO4·xH2O/graphene composite exhibited the more outstanding electrochemical performances than that of FeVO4·xH2O.Based on the above experimental results,the FeVO₄·xH₂O/graphene composite could be a promising anode material for lithium-ion batteries.（2）The FeVO₄ sample was synthesized through a hydrothermal and heat treatment method by using Fe（NO₃）₃·9H₂O and NH₄VO₃ as raw materials.The calcination condition was that the product was heated at 500°C for 4 h at the heating/cooling rate of 5°C/min in the air atmosphere.By being combined with graphene,the FeVO₄/graphene composite was obtained.XRD,TG,Raman,XPS,SEM and TEM tests were conducted to analysize the structure,thermostability,element composition and morphology.The formation process of the prepared samples was also explained.The results showed that the prepared samples took triclinic type FeVO4 as main structure and owned the morphology of microrods.After being combined with graphene,the crystallinity of the sample got improved markedly and the microrods were evenly anchored in the internet of graphene.Besides,the size of the composite decreased significantly.The electrochemical properties were tested by galvanostatic charge/discharge system and electrochemical workstation.In addition,the electrochemical reaction mechanism was also studied.The discharge specific capacity of FeVO₄ and FeVO₄/graphene composite still maintained at 355.4 mAh g^-1and 1042.3 mAh g^-1respectively after 100 cycles at the current density of 100 mA g^-1 between 0.01 and 3.0 V versus Li/Li⁺.Via comparison,the FeVO₄/graphene composite behaved the lower charge transfer resistance and the higher Li⁺diffusion coefficient.Therefore,the FeVO₄/graphene composite exhibited the more outstanding electrochemical performances than that of FeVO₄.Based on the above experimental results,the FeVO₄/graphene composite could be a promising anode material for lithium-ion batteries.（3）The Zn₃V₂O₇（OH）₂?2H₂O sample was synthesized through a solvothermal route by using Zn（CH3COO）2,VO（ac）2 and DMF as raw materials.Solvothermal time was setted as 3h,6 h,12 h,18 h,24 h and 30 h,corresponding products were marked as sample-1,sample-2,sample-3,sample-4,sample-5 and sample-6 respectively.XRD,TG,SEM and TEM tests were conducted to analysize the structure,thermostability and morphology.The formation process of the prepared samples was also explained.The results showed that the prepared samples took hexagonal type Zn₃V₂O₇（OH）₂?2H₂O as main structure and owned the morphology of microspheres.With prolonging of the solvothermal time,the crystallinity of the samples got better and the morphology of the microspheres became more inerratic and integrated.Among the prepared samples,sample-5 showed the best crystallinity and the perfectest morphology.The electrochemical properties were tested by galvanostatic charge/discharge system and electrochemical workstation.In addition,the electrochemical reaction mechanism was also studied.The discharge specific capacity of sample-1,sample-2,sample-3,sample-4,sample-5and sample-6 still maintained at 534.5 mAh g^-1,726.2 mAh g^-1,896.4 mAh g^-1,1049.8 mAh g^-1,1281.4 mAh g^-1 and 1097.3 mAh g^-1respectively after 100 cycles at the current density of100 mA g^-1 between 0.01 and 3.0 V versus Li/Li⁺.Via comparison,sample-5 exhibited the highest capacity retention and behaved the lowest charge transfer resistance and the highest Li⁺diffusion coefficient.Therefore,sample-5 exhibited the most outstanding electrochemical performances than those of the other five samples.Thus,24 h was proved as the optimal solvothermal time for the synthesis of Zn₃V₂O₇（OH）₂?2H₂O microspheres.Based on the above experimental results,the Zn₃V₂O₇（OH）₂?2H₂O sample could be a promising anode material for lithium-ion batteries.（4）Zn₃V₂O₈ sample was obtained by calcining the Zn₃V₂O₇（OH）₂?2H₂O（24 h）sample.The calcination condition was that the Zn₃V₂O₇（OH）₂?2H₂O product was heated at 500°C for2 h at the heating/cooling rate of 5°C/min in the air atmosphere.XRD and SEM tests were conducted to analysize the structure and morphology.The results showed that the prepared sample took orthorhombic type Zn₃V₂O₈ as main structure and owned the morphology of sheets.The electrochemical properties were tested by galvanostatic charge/discharge system and electrochemical workstation.In addition,the electrochemical reaction mechanism was also studied.The discharge specific capacity of Zn₃V₂O₈ still maintained at 872.4 mAh g^-1respectively after 100 cycles at the current density of 100 mA g^-1 between 0.01 and 3.0 V versus Li/Li⁺,which exhibited excellent cycling and rate performances.Based on the above experimental results,the Zn₃V₂O₈ sample could be a promising anode material for lithium-ion batteries.（5）The Zn₃V₃O₈ sample was synthesized through a solvothermal method by using Zn（NO3）2·6H2O,VO（ac）2 and DMF as raw materials.Solvothermal time was setted as 3 h,6h,12 h,18 h and 24 h,corresponding products were marked as sample-1,sample-2,sample-3,sample-4 and sample-5 respectively.XRD,TG,XPS,SEM and TEM tests were conducted to analysize the structure,thermostability,element composition and morphology.The formation process of the prepared samples was also explained.The results showed that the prepared samples took cubic type Zn3V3O8 as main structure and owned the morphology of microspheres.With prolonging of the solvothermal time,the crystallinity of the samples got better and the microspheres of the samples became more inerratic and integrated.However,when the reaction time was 24 h,the morphology of some microspheres was damaged.Among the prepared samples,sample-4 showed the best crystallinity and the perfectest morphology.The electrochemical properties were tested by galvanostatic charge/discharge system and electrochemical workstation.In addition,the electrochemical reaction mechanism was also studied.The results showed that the discharge specific capacity of sample-1,sample-2,sample-3,sample-4 and sample-5 still maintained at 511.1 mAh g^-1,612.7 mAh g^-1,896.9 mAh g^-1,1033.0 mAh g^-1and 935.1 mAh g^-1 respectively after 100 cycles at the current density of 100 mA g^-1 between 0.01 and 3.0 V versus Li/Li⁺.Via comparison,sample-4exhibited the highest capacity retention and behaved the lowest charge transfer resistance and the highest Li⁺diffusion coefficient.Therefore,sample-4 exhibited the most outstanding electrochemical performances than those of the other four samples.Thus,18 h was proved as the optimal solvothermal time for the synthesis of Zn₃V₃O₈ microspheres.Based on the above experimental results,the Zn3V3O8 sample could be a promising anode material for lithium-ion batteries.